C6 is the heavy general overhaul.....usually carried out every 10 years.
Not quite - C6 is a body repair which is carried out to a time periodicity. The scope can vary, but usually includes attention to things above the sole bar, including corrosion, windows, gangways, paint etc.
A C4 is a mileage based exam for the running gear and mechanical components primarily below the solebar (eg bogies, brake gear, couplers) and is undertaken on completion of the prescribed number of miles operated.
A C3 is a combination of a C4 and C6 undertaken simultaneously which occurs on a few fleets where mileage operated approximately equates to time between body overhauls.
C2 and C1 are effectively half life or major rebuilds, whilst a C5 is unscheduled work. These classifications are rarely used now.
The work on a C6 does NOT include interior work, although this is often undertaken in conjunction with a C6 programme. Interior work can be as great or as lilttle as the operator is prepared to pay for or deems necessary, hence the difference in levels between Major Refurbishments (GNER Mallard) and Refreshes (NX Mark IIIs).
Hope this helps